The present invention generally relates to processes and apparatuses used in the treatment of materials. More particularly, this invention relates to processes and apparatuses for producing porous media, such as nano-porous silicon (npSi) suitable for use in the storage and retrieval of elemental hydrogen.
Hydrogen-based fuel cell technologies are being considered for a wide variety of power applications, including but not limited to mobile applications such as vehicles as an attractive alternative to the use of petroleum-based products. Hydrogen-based fuel cells are also readily adaptable for use as energy sources in numerous and such diverse applications as cellular phones to space ships. They have the further desirable attribute of producing water vapor as their only byproduct and are thus environmentally benign.
Efficient storage of hydrogen is vitally important for cost-effective system implementation. When compared to storage for conventional chemical fuels or electric energy sources, existing hydrogen storage technologies lack the convenience of gasoline for delivery and storage capacity (energy density per unit weight), and lack the flexibility of electrical energy stored in batteries and capacitors. Therefore, for fuel cells to reach their full commercial potential, improved hydrogen storage technologies are needed.
Prior methods of storing hydrogen fall broadly into two categories. The first category involves storing hydrogen chemically within a convenient chemical molecule, usually an aliphatic organic compound such as methane, octane, etc., and then pre-processing the fuel as needed, such as by catalytic reforming, to release elemental hydrogen plus carbon oxides. This method suffers two important drawbacks: carbon dioxide byproduct is a “greenhouse gas” that some believe contributes to global warming and is therefore environmentally undesirable; and the additional weight of the chemical molecule and the reformer reduce the efficiency of the entire process, making it less attractive from a cost and performance standpoint.
The second category involves mechanical or adsorptive storage of elemental hydrogen in one of three forms: compressed gas, cryogenically-refrigerated liquid, or chemisorbed onto active surfaces. Of these methods, compressed gas storage is the most straightforward and is a mature technology. However, compressed gas cylinders are quite heavy, needing sufficient strength to withstand pressures of many thousands of pounds per square inch. This weight is a considerable drawback for portable applications, and in any usage compressed gas cylinders must be treated with care because they represent a safety hazard.
Cryogenic storage of hydrogen is also well known, being used in industrial plants and as a rocket fuel. Liquid hydrogen is remarkably dense from a specific energy point of view (kilowatts per kilogram), but requires a considerable amount of additional energy to maintain the nearly absolute zero temperatures needed to keep hydrogen in a liquid state. Liquid hydrogen also requires a heavy mass of insulation, and these factors conspire to make cryogenic storage impractical for portable and small-scale applications.
Chemisorption as used herein means the adsorption of a given molecule onto an active surface, typically of a solid or a solid matrix. Chemisorption is typically reversible, although the energy of adsorption and the energy of desorption are usually different. Various catalysts and surface preparations are possible, providing a wide range of possible chemistries and surface properties for a given storage problem. Chemisorption of hydrogen has been studied extensively, and substances such as metal hydrides, palladium, and carbon nanotubes or activated carbon have been used to adsorb and desorb hydrogen.
Prior hydrogen chemisorption techniques have fallen short of the goals of efficiency, convenience, and low system cost for several reasons. In some materials, such as carbon nanotubes, the efficiency of hydrogen adsorbed per unit weight of matrix is moderate, but the method of desorption requires high heat, which brings about danger of combustion. Additionally, the present cost of carbon nanostructures is relatively high, and control over material properties can be quite difficult in high-volume manufacturing. In the case of metal hydrides, metal oxides, and other inorganic surfaces, storage efficiencies typically are lower and the adsorption/desorption process is highly dependent upon exacting chemistry. These factors combine to make such approaches less than sufficiently robust for many commercial applications.
Hydrogenated surfaces in silicon have also been employed, as disclosed in U.S. Pat. Nos. 5,604,162, 5,605,171, and 5,765,680, the disclosures of which are incorporated herein by reference. In each of these references, the adsorbed molecule is the radioactive hydrogen isotope tritium (3H), and the objective is the storage of this isotope to enable its safe transport, typically to a waste handling or storage facility, or to serve as a means for providing radioactive energy to power a light source. These prior methods of chemisorption do not, however, provide for desorption of hydrogen from a silicon storage medium. In fact, conventional methods of chemisorption are generally designed to prevent desorption. Further, these conventional methods of chemisorption fail to teach methods by which the storage capacity of a silicon matrix can be increased.
As a solution to the forgoing, a system for storage and retrieval of elemental hydrogen on a porous silicon media is described in U.S. Published Patent Application No. 2004/0241507 to Schubert et al., the disclosure of which is incorporated herein by reference. Prior to Schubert et al. and contemporaneous research, the most widely known applications for nano-porous silicon (npSi) concerned the emission of light. Silicon, an indirect band gap semiconductor, emits light in such small quantities under normal conditions that optical devices such as light-emitting diodes and lasers are not made of silicon. However when silicon is made porous, it fluoresces (emits light) under exposure to ultraviolet light.
A very large number of technical papers describe methods of making npSi using an electrochemical etch. A common starting configuration for making npSi is a silicon wafer, such as is used in the semiconductor industry. For the npSi reaction to proceed with an electrochemical etch, the wafer must contain holes (carriers), which can be introduced by p-type doping, photogeneration, etc. The wafer, or a portion thereof, is clamped in a fixture, attached to an electrode, and then one side of the wafer is exposed to an etchant solution while electricity passes through the wafer. The etching process produces a npSi layer in the surface of the wafer exposed to the etchant solution. FIG. 1 schematically represents a standard porous silicon electrochemical reaction cell 10, with which an electrical bias is placed across a silicon substrate 12 while the substrate 12 is exposed to an etch bath 14 to form a npSi layer 16 in the negatively biased surface of the substrate 12. Electrochemical methods of the type represented in FIG. 1 have not been optimized for large quantities or high surface areas, i.e., small pore sizes.
As evident from FIG. 1, npSi formation is largely a surface phenomenon. Layer thicknesses are also somewhat self-limiting, because the outermost portion of the npSi layer may etch away as npSi forms at the reaction front beneath the outermost portion. Additionally, the reaction front moves more slowly into the substrate as the aspect ratio of the pores becomes higher, limited by the transport of reactants into the pores and reaction products out of the pores. Once a layer of npSi is formed on a substrate, it must be removed intact from the substrate to produce free npSi.
There exist applications of npSi that would benefit from npSi being formed on silicon particles instead of silicon wafers. In fact, using appropriate etch conditions and particle sizes, it would be possible to completely transform silicon particles from a fully dense single crystalline structure into a completely porous npSi structure, which would eliminate the need to free a npSi layer from a large substrate. However, while successfully used to form npSi using wafers as a starting material, it is impractical to electrochemically etch small particles. For example, it would be extraordinarily difficult to provide for a bias across an individual particle. Placing the particles in a solution and providing a bias across the solution would not result in a bias across each particle unless the solution was a very electrically resistive solution. Every surface of a particle immersed in the solution would be at the same potential, and therefore no net bias would be applied across any particle.
A purely chemical method of making npSi, often referred to in the literature as a “stain etch,” is also known. Stain etching is not an electrochemical etch and thus does not require an electrical bias, which potentially makes it more practical than electrochemical etching for making npSi on silicon particles and powders. The stain etch method of producing npSi is slow and, as with electrochemical etching, requires holes for the npSi reaction to proceed. Also similar to electrochemical etch processes, known stain etch methods are not applicable to bulk manufacture of high surface area npSi on silicon particles or powders.
In view of the above, there is an ongoing need for processes capable of producing large amounts of free, extremely high surface area npSi.